This invention relates in general to resonators and relates more particularly to thin film YIG resonators. Yittrium Iron Garnet (YIG) spheres are used as resonator elements in many microwave components and in particular are used in tunable oscillators. The solid state active devices used in these units are typically mounted on a suitable transmission medium and the YIG spheres are mounted on a separate support rod. Energy is exchanged between the active device and the YIG sphere via a coupling loop that is electrically connected to the active device and that is magnetically coupled to the YIG sphere.
Unfortunately, there are several disadvantages to this type of resonator. Because the YIG sphere and the active device are not mounted on the same support surface, this device can exhibit significant temperature dependent behavior. The YIG sphere resonator cannot be manufactured by the photolithographic technology typically used to manufacture the active circuit so that the cost associated with circuits utilizing YIG spheres is increased. The sphere to loop alignment and the loop to transmission medium attachment are critical parameters that further increase fabrication expense and lead to variable resonator quality. The YIG sphere utilizes a resonant mode in which the magnetic dipoles in the sphere precess around a common axis that is oriented along the direction of an applied bias magnetic field. Ideal operation of the YIG shperes require that the spheres be perfectly elliptical. Any asymmetry in the sphere, coupling loop or bias field will result in spin wave interference with the uniform procession mode, thereby degrading the performance of the YIG sphere resonator.
Several forms of magnetostatic wave (MSW) resonators are also known. These tunable resonators can be used as the frequency selective elements in tunable oscillator circuits in the 0.5-26 GHz frequency range. The planar structure of the resonators makes integration with other integrated circuits easy and attractive. Moreover, because the velocity of propagation of magnetostatic waves in YIG films is only two order of magnitudes lower than the velocity of light, the linewidths involved in fabricating magnetostatic wave (MSW) resonators are of the order of 10-100 microns, allowing easy photolithographic processing.
The first MSW resonator reported in the literature (J. H. Collins, J. D. Adam and Z. M. Bardai, "One-port magnetostatic wave resonator", Proc. IEEE, July 1977, pp. 1090-1092) utilizes a 9 micron YIG film that is etched to form an array of parallel equispaced grooves that are 30 microns wide, 4000 microns long 1 micron deep and spaced by 120 microns. A microstrip transducer is formed over the region of the YIG film containing the grooves. These grooves each reflect some of the magnetostatic waves generated by the transducer and have a resonant response when the wavelength of these waves equals the spacing of the etched grooves. Unfortunately, it is difficult to etch accurate grooves in the YIG film. In addition, these etched grooves exhibit a high amount of loss.
A two port resonator has also been produced (see W. R. Brinlee, J. S. Owens, C. V. Smith, Jr. and R. L. Carter, "Two-port magnetostatic wave resonators utilizing periodic metal reflective arrays", J. Appl. Phys., Vol. 52, Noi. 3, March 1981, pp. 2276-2278) that utilizes a metal reflective array on the surface of the YIG film instead of an array of etched grooves. Because of excessive eddy current losses in these metal arrays, the insertion loss of these resonators is more than 30 dB and the loaded Q is about 600.
In U.S. Pat. No. 4,528,529 entitled "Magnetostatic Wave Resonator" by Ernst Huijer is presented a resonator that has a block of YIG film that has two sides that are both substantially parallel to a microstrip transducer or to a pair of parallel microstrip transducers.